An air battery in which oxygen is used as an active material, has many advantages such as high energy density. Well-known examples of air batteries include metal-air batteries such as an iron-air battery and an aluminum-air battery.
As a technique relating to such air batteries, an iron-air battery including a cathode (air electrode), an electrolyte and an anode containing an iron metal, is disclosed in Patent Document 1 (Japanese Patent Application Laid-Open No. 2012-094509), for example.
However, the surface of the iron anode used in an iron-air battery is generally covered with an oxide layer (passivation layer) and problematic in that the iron anode is inactive as a battery electrode. To remove the passivation layer, generally, a reduction treatment is carried out before discharge. The surface of the iron anode is activated by this treatment.
However, there is a problem as follows: due to the influence of dissolved oxygen, hydroxides and so on in the electrolyte, the iron anode surface is re-passivated just after the reduction treatment and makes it difficult to obtain discharge capacity.
To prevent the iron anode surface from re-passivation, it is disclosed in Patent Document 1 that a decrease in electrode activity can be inhibited and a discharge reaction can be promoted by adding potassium sulfide (K2S) to the electrolyte.
In the case of using the electrolyte containing K2S, the discharge capacity of the iron-air battery can be increased. However, the discharge capacity significantly changes as the concentration of K2S changes. Accordingly, there is such a problem that highly accurate concentration control is necessary to obtain desired discharge capacity.